The invention relates to a monolithically integratable transistor circuit for limiting transient positive high voltages on an electric conductor, including a pnp transistor having an emitter terminal connected to the electric conductor and a collector terminal connected to a reference potential, the pnp transistor being conductive only upon exceeding a predetermined potential threshold value being positive as compared with the reference potential. The invention also relates to semiconductor structures for simple attainment of such transistor circuits.
German Published, Non-Prosecuted Application DE-OS No. 26 54 419 discloses circuit configurations for voltage limitation on electric conductors, in which a controllable path of a semiconductor array, in particular the collector-to-emitter path of a transistor, is disposed between the conductor to be protected and reference potential. From the control input of the semiconductor array, diode chains are connected in the blocking direction with respect to the conductor to be protected and to reference potential, respectively. In order to provide protection against positive high voltages on the conductor to be protected, the corresponding diode chain is connected in the conducting direction from the conductor to be protected to the control input of the semiconductor array. The number of diodes determines the maximum allowable voltage. However, in integrated circuits such diode chains should be avoided if at all possible, because they require so much surface area. Furthermore, the voltage value at which the voltage limitation is supposed to be initiated is only imprecisely defined, and the impedance with which such a circuit loads the conductor to be protected is subject to relatively major variation, and considerably limits the processable signal frequency.
German Published, Non-Prosecuted Application DE-OS No. 31 25 198, in particular FIG. 3 thereof, discloses a transistor protection circuit for protecting electric conductors against transient positive high voltages, in which the emitter of a pnp transistor is connected to the conductor to be protected, the collector of the pnp transistor is connected to the reference potential, and the base terminal of the pnp transistor is connected to the reference potential through a diode in the conducting direction and is acted upon by a reference bias voltage. If, for instance, the conductor to be protected is connected to a bus or port of a data system, then it is possible for both signal voltages and ESD pulses to appear on the conductor even if the semiconductor circuit to be protected, including the transistor protection circuit, is not in operation. However, in such a case, a transistor protection circuit of the kind known from German Published, Non-Prosecuted Application DE-OS No. 31 25 198, would also not be in operation, because no reference bias voltage is present.
The use of substrate diodes is a proven way to protect electric conductors against transient negative high voltages. A protective circuit known from German Published, Non-Prosecuted Application DE-OS No. 33 01 800 for protecting against negative high voltages has better high-frequency properties.